The present invention relates to sintered, substantially round and spherical particles and methods for producing such sintered, substantially round and spherical particles from a slurry of an alumina-containing raw material. The methods described herein provide for nucleation and promotion of the growth of mullite crystals in the sintered particles, which in turn contributes strength to the sintered particles. The sintered particles are prepared from a slurry of a raw material having an alumina content of from about 40% to about 55% by weight, and a metal oxide sintering aid in an amount of from about 1% to about 6% of the dry weight of the total solids in the slurry. The sintered particles are suitable for use as a propping agent or as a foundry media.
Oil and natural gas are produced from wells that penetrate porous and permeable subterranean formations. The porosity of such a formation permits the formation to store oil and gas, and the permeability of such a formation permits the oil or gas to move through the formation. The permeability of such a formation is essential to permit oil and gas to flow to a location where it can be pumped from the well. Sometimes, however, the permeability of a formation is insufficient for the economic recovery of oil and gas. In other cases, during operation of the well, the permeability of a formation decreases to the extent that further recovery of oil and gas becomes uneconomical. In such cases, it is necessary to fracture the formation and prop the fracture in an open condition by means of a proppant material or propping agent. Such fracturing is usually accomplished by hydraulic pressure, and the proppant material or propping agent is a particulate material, such as sand, glass beads or ceramic particles, which are carried into the fracture by means of a fluid. The use of low density proppant has generally been limited to use in reservoirs with low to moderate closure stress (2000-7500 psi) due to the inherent lack of strength of low density proppant at higher closure stress relative to high density bauxite-based proppants. A lightweight proppant which has strength, long term conductivity and long term permeability equal to or greater than intermediate or high density proppants at closure pressures about 6000 psi would be beneficial.
The foundry industry has traditionally used inexpensive natural sands for casting. However, natural sands exhibit large thermal expansion characteristics, which means that the core boxes and molds must be highly pre-engineered to allow for the expansion of the sand when it comes in contact with the heat from the molten metal. Many times, despite pre-engineering efforts, expansion related defects occur resulting in scrapped parts or parts requiring high cleaning cost to correct. Also, natural sands contain crystalline quartz silica, which is a health and environmental hazard within the workplace. In addition, natural sands are mined, which results in product inconsistencies that lead to casting inconsistencies. Furthermore, most silica sands have a low degree of roundness and are somewhat friable resulting in low flowability and compaction, high breakdown and high solid waste emissions. Specialty sands, such as zircon and chromite, have low expansion properties but are very dense, making core handling more difficult and requiring more resin to obtain adequate tensile strengths. These products are mined and present concerns of availability, consistency and sizing flexibility. They also tend to be somewhat friable.